Improvement in Facebook Grabber

_ GRKFacebookConnector : Do not assume the connection is valid only
according to the session's state : also test the connection. the user
may have rejected the FB app or changed his password and logged out.
_ GRKFacebookGrabber : better handling of errors in a batch request
Update in GrabKit Demo's pbxproj

Some frameworks must be linked as "weak" for iOS 5.1
Fix for issue #12 : It's now possible to detect if an authentication process has not been completed

Update in all the connectors + impact on the demo + impact in
documentation
Project update for Xcode 4.6 + cleanup
Update in changelog and readMe for GrabKit v1.2.3